1. Field of the Invention
The present invention relates to the field of pressure measuring instruments, particularly those instruments capable of accurately measuring very low gas pressures. More specifically it relates to the type of instrument known as a Penning or cold-cathode ionization vacuum meter.
2. Description of Prior Art
Cold-cathode ionization or Penning vacuum meters are known in the art. This type of vacuum meter operates on the principle that the rate of ion production by a stream of electrons in a vacuum system is dependent on the pressure and ionization probability of the residual gases. The meter is generally comprised of two cathodes arranged opposite one another with an anode centrally spaced between them, the cathodes and anode being mounted inside a metal envelope, the envelope being connected to the atmosphere being measured. Outside the envelope a permanent magnet provides a magnetic field to lengthen the path travelled by the electrons going from the cathode to anode, which increases the amount of ionization, allowing the meter to operate well at low pressures. The anode is normally set to a potential of 2 kV, with gives rise to a direct current caused by positive ions arriving at the cathode. The pressure is indicated directly by the magnitude of the current.
At very low pressures this type of vacuum meter frequently fails to operate properly as there is insufficient ionization in the residual gases for measurement operations to begin. Several methods have been used in attempts to correct this problem.
German patent DE-OS No. 30 47 744 uses an additional host cathode, in addition to the normal cold cathode and anode, to insure sufficient ionization in the residual atmosphere. This method requires the use of additional power sources, as the hot cathode must be provided with a large current.
Various other suggestions, such as a metal point in the discharge chamber or the use of ionizing radioactive materials in the chamber have been made. Either they did not lead to the desired results (the metal points), or they caused additional problems (radioactive materials, e.g.).
Consequently, there is still a need for some device or improvement to be used with a cold-cathode ionization vacuum meter which will insure reliable low pressure operations.